JP2002328117A - Microphone for photoacoustic gas sensor, and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a microphone for a photoacoustic gas sensor of simple structure capable of enhancing detection sensitivity of the photoacoustic gas sensor. SOLUTION: This sensor is provided with the first corrugated electrode 21 provided to form one portion of a wall face of a cavity 10, and the second electrode supported by a backplate 23 contacting adjacently with the cavity and arranged opposedly with a prescribed distance in a reverse face side of the first electrode. In particular, the first electrode is formed using a corrugated shape provided by anisotropic-etching one face of an Si substrate 11 for forming the cavity, the second electrode is formed thereon via sacrifice layer 16, and the backplate is formed further. The Si substrate is selectively etched from its reverse face side, the first electrode is thereby exposed to form the cavity, and the sacrifice layer is etchedly removed to complete the microphone.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microphone for a photoacoustic gas sensor incorporated in a photoacoustic gas sensor and responsive to a pressure in a cavity into which a gas is introduced, and a method of manufacturing the same.

[0002]

[Related Background Art] A photoacoustic gas sensor utilizes a phenomenon in which a specific type of gas absorbs infrared rays of a specific wavelength and thermally expands, thereby using a specific type of gas in a mixed gas such as air.
For example, it detects the concentration of CO ₂ . That is, when air is irradiated with infrared light having a specific wavelength and the intensity of which changes with time, the greater the CO ₂ concentration in the air, the greater the thermal expansion and contraction. Therefore, if this phenomenon is detected as a change in atmospheric pressure (sound pressure), it becomes possible to detect the CO ₂ concentration in the air. Incidentally, this type of photoacoustic gas sensor basically has a cavity 1 into which gas is introduced as shown in FIG.
The light source 2 includes a light source 2 for intermittently irradiating a pulse of infrared light, and a microphone 3 which forms part of a wall surface (ceiling surface) of the cavity 1 and is sensitive to a sound pressure in the cavity 1.

Recently, attempts have been made to reduce the size of this type of photoacoustic gas sensor by applying semiconductor device manufacturing technology. In this case, the cavity 1 has a structure in which a predetermined space is formed by etching a Si substrate transparent to infrared light, and a gas passage 4 for introducing a gas into the cavity 1 is provided. Be produced. The gas passage 4 is usually provided with a gas diffusion filter 5. The gas diffusion filter 5 has a large number of fine ventilation holes, and maintains the flow (replacement) of gas (air) between the inside and the outside of the cavity 1 by restricting the flow of gas. However, it plays a role of changing the sound pressure in the cavity 1 according to the thermal expansion of the gas due to the absorption of the infrared light described above.

[0004] The gas diffusion filter 5 has the following two roles. One of them is to act as a large airflow resistor against a sudden pressure change (sound pressure) generated in the cavity 1 due to the irradiation of the infrared pulse, and to maintain the inside of the cavity 1 in a substantially sealed state, thereby maintaining the sound pressure. Is a function of preventing the light from transmitting to the outside of the cavity 1. The other is that it does not act as an airflow resistor against a slow pressure change (sound pressure) in the cavity 1 due to a change in the external environment such as temperature or atmospheric pressure. It is a function to keep it open.

[0005]

By the way, as the size of the photoacoustic gas sensor is reduced, the optical path length in the cavity 1 of the infrared light which is intermittently modulated and emitted from the light source 2 tends to be shortened. For this reason, the amount of infrared light absorbed by the gas decreases, and the gas does not thermally expand sufficiently, so that it cannot be denied that the change in sound pressure becomes small. That is, the detection sensitivity for the gas concentration is deteriorated. To compensate for this, for example, it is necessary to increase the sensitivity of the microphone 3 and detect a change in sound pressure generated in the cavity 1 with high sensitivity. However, there is a limit in increasing the sensitivity of the microphone 3, and even if the sensitivity of the microphone 3 is increased, it is unavoidable that a new problem such as noise resistance occurs.

The present invention has been made in view of such circumstances, and has as its object to provide a microphone provided as a part of a cavity into which gas is introduced, and a sound pressure generated in the cavity. It is an object of the present invention to provide a microphone for a photoacoustic gas sensor having a simple configuration capable of reliably detecting a change in the photoacoustic gas and enhancing the gas detection sensitivity of the photoacoustic gas sensor, and a method of manufacturing the same.

[0007]

In order to achieve the above-mentioned object, a microphone for a photoacoustic gas sensor according to the present invention is provided as a part of a cavity into which a gas is introduced, and is responsive to the pressure in the cavity. A wavy first electrode provided as a part of a wall surface of the cavity, and a predetermined distance from the first electrode supported by a back plate connected to the cavity. And a second electrode that is opposed to the second electrode. Note that the second electrode may be provided outside the cavity, or may be provided inside the cavity.

Preferably, the cavity is formed by forming a through hole having a predetermined opening area in a semiconductor substrate, and the first electrode closes one of the openings of the through hole. It is provided so as to form a part of the cavity (claim 2). Further, it is preferable that the first electrode or the back plate includes an infrared light reflecting film such as Au coated on a surface exposed to the cavity.

Further, according to the method of manufacturing a microphone for a photoacoustic gas sensor according to the present invention, one surface of a Si substrate or the like as a semiconductor substrate is anisotropically etched, for example, using a mask having a predetermined opening pitch. Is processed into a corrugated shape (first step), and the Si processed into this corrugated shape is processed.
After forming the first electrode along one surface of the substrate (second step), a sacrificial layer having a predetermined thickness is formed on the upper surface of the first electrode (third step). Next, after a second electrode is formed on the sacrificial layer (fourth step), a back plate is formed so as to cover the second electrode, and a predetermined opening is formed in the back plate to form an acoustic hole. It is formed (fifth step). Then, the Si substrate is selectively etched from its rear surface side to expose the first electrode (sixth step), and the sacrificial layer is removed by etching through the acoustic hole (seventh step). ), Forming a microphone for a photoacoustic gas sensor integrated with the cavity.

[0010] Further, the surface of the first electrode or the back plate exposed to the cavity is coated with an infrared light reflection film such as Au (eighth step).

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A microphone for a photoacoustic gas sensor according to an embodiment of the present invention and a method for manufacturing the same will be described below with reference to the drawings. This microphone for a photoacoustic gas sensor is manufactured by disassembling the manufacturing process in FIGS. 1 and 2, and finally, as shown in FIG. It is implemented as a part and is responsive to the pressure in the cavity 10. In particular, the microphone 20 has a first electrode 21 forming a part of the cavity 10 having a corrugated shape, and a second electrode 2 forming a pair with the first electrode 21.
2 is supported by a back plate 23 connected to the cavity 10, and has a structure in which the second electrode 2 is opposed to the back surface of the first electrode 21 at a predetermined distance.

The details of the microphone 20 having such a characteristic structure will be described below in accordance with the manufacturing process. The microphone 20 is formed directly on the Si substrate 11 forming the cavity 10. Specifically, an Si substrate 11 for forming the cavity 10 is prepared. As the Si substrate 11, for example, (10
A Si wafer having the 0) plane as a main surface is used. First, a thermal oxide film 12 is grown on the Si (100) substrate 11, and the thermal oxide film 12 is formed by photolithography.
Then, as shown in FIG. 1A, rectangular holes 13 are formed at a predetermined opening pitch.

The rectangular hole 1 provided in the thermal oxide film 12
For example, as shown in FIG. 3 (a), small holes 3 may be arranged vertically and horizontally at equal intervals, or rectangular holes may be formed as shown in FIG. 3 (b). May be provided in parallel, or as shown in FIG. 3 (c), a frame shape in which the shape of the hole is gradually elongated from the center to the outside. In short, as described later, when the Si substrate 11 is anisotropically etched using the thermal oxide film 12 as a mask, a mask pattern may be formed so that unevenness having a uniform waveform shape is formed on the surface thereof. .

Thereafter, using the thermal oxide film 12 as a mask, the Si substrate 11 is anisotropically etched using KOH or TMAH (trimethylammonium hydride). Then, as shown in FIG.
A plurality of concave portions 14 having a triangular cross section are formed on the surface of the
Then, a waveform defining the shape of the first electrode 21 at 0 is formed on the surface of the Si substrate 11 [first step].

Next, the thermal oxide film 1 used as the mask
1 is removed, and then the Si substrate 1 is removed as shown in FIG.
1 to protect the surface of the Si substrate 11 and to insulate the first electrode 21 from the Si substrate 11 over the entire surface of the Si substrate 11.
A thermal oxide film (not shown) such as O ₂ is formed. Then, on the thermal oxide film (SiO ₂ ), a first electrode 21 made of, for example, Cr / Au / Cr is formed to a thickness of 4/20/4 nm, and a photosensitive film used as a diaphragm is formed on the first electrode 21. A polyimide (not shown) is formed to a thickness of 1 μm. That is, along one surface of the Si substrate 11 processed into the corrugated shape,
The first electrode 21 lined with the diaphragm is insulated from the Si substrate 11 to form a corrugated shape [second step].

Thereafter, on the upper surface of the first electrode 21,
As shown in FIG. 1D, Al serving as the sacrificial layer 16 is formed to a thickness of 2 μm. The sacrificial layer (Al) 16 plays a role in defining a distance (gap) between the second electrode 22 and the first electrode 21 in the microphone 20, and the thickness thereof is reduced by vacuum evaporation or sputtering. It is formed while controlling [third step].

Thereafter, as shown in FIG. 1E, a second electrode 22 made of, for example, Au / Cr is formed on the sacrificial layer (Al) 16, and the second electrode 22 is covered with the second electrode 22. 1 (f)
As shown in (4), a photosensitive polyimide forming the back plate 23 is formed to a thickness of about 10 to 20 μm [fourth step]. The second electrode 22 is made of Au / Cr in consideration of the resistance at the time of removing the sacrificial layer 16 by etching, as described later, but the sacrificial layer (Al) is used here.
Of course, other electrode materials can be used as long as they have resistance to the 16 etching solutions.

Thereafter, a predetermined opening is formed by patterning the back plate 23 made of a photosensitive polyimide, and the second electrode 22 is selectively etched by using the back plate 23 as a mask, thereby obtaining a structure shown in FIG. As shown in (5), a plurality of acoustic holes 24 that function to release air from the microphone 20 are formed [fifth step]. These acoustic holes 24 are also used for etching and removing the sacrificial layer (Al) 16 described above.

Next, a rectangular hole is formed in the thermal oxide film 17 remaining on the back surface of the Si substrate 11 by using photolithography. Then, using the thermal oxide film 17 as a mask, the Si substrate 11 is anisotropically etched from the back side using KOH or TMAH (trimethylammonium hydride) to expose the first electrode (diaphragm) 21. As shown in FIG. 2 (b), a space forming the cavity 10 is formed inside the Si substrate 11 [sixth step]. At this time, Au is sputtered into the cavity 10 from the back surface side of the Si substrate 11 so that a thickness of several μm is formed on the exposed surface of the first electrode (diaphragm) 21 and the inner wall surface of the Si substrate 11 where the cavity 10 is formed. It is desirable to form the infrared light reflection film 18 [eighth step].

Thereafter, through the acoustic hole 16 shown in FIG.
As shown in (c), the sacrificial layer 16 is removed by etching.
By forming a predetermined gap (space) between the first electrode 21 and the second electrode 22 [seventh step], the microphone 20 integrated with the acoustic cell (cavity) of the photoacoustic gas sensor is formed. Be completed. Incidentally, the sacrificial layer 16 made of Al is removed by etching, for example, using a mixed solution of phosphoric acid and nitric acid (70 ° C.) as the etching solution.

For the acoustic cell, an optical filter substrate (not shown) is joined to the back side of the Si substrate 11 where the opening is formed to complete the cavity 10, and infrared light is introduced through the optical filter substrate. What is necessary is just to make it possible to configure. However, as shown in FIG.
Similarly to the above, another Si substrate 19 in which a space forming the cavity 10 is formed by anisotropic etching is joined to the back surface side of the Si substrate 11 to realize an acoustic cell in which a larger space (cavity) is formed. It is possible. In this case, an optical filter substrate may be bonded to the open surface of the Si substrate 19. In particular, with such a structure, it is possible to increase the optical path length of the infrared light in the cavity 10. When the two Si substrates 11 and 19 are joined to form the cavity 10 in this manner, the spacer 3 having the gas diffusion filter formed therebetween is formed.
0 may be interposed.

According to the microphone 20 manufactured as described above and having the waveform of the first electrode 21 forming a part of the cavity 10, the area of the first electrode 21 receiving the sound pressure in the cavity 10 is formed. Can be broadened, so that the sensitivity can be effectively increased. Moreover, the cavity 10 of the first electrode (diaphragm) 21
Since the exposed surface on the side is coated with the infrared light reflection film (Au) 18, the first electrode (diaphragm) in which the infrared light introduced into the cavity 10 forms a waveform.
It is reflected by 21. In combination with the infrared light reflecting film (Au) 18 coated on the inner wall surface of the cavity 10 formed by the Si substrates 11 and 19, the infrared light repeats multiple reflections in the cavity 10 and substantially. It is possible to increase the optical path length in each step.

As a result, the amount of infrared light absorbed by the gas (eg, CO ₂ ) introduced into the cavity 10 increases, and the thermal expansion of the gas can be increased, so that the change in the sound pressure is increased. It becomes possible. In other words, even if the concentration of the gas is low, the change in sound pressure can be increased, so that the detection sensitivity of the acoustic cell can be effectively increased.

According to the above-described manufacturing method, the first electrode (diaphragm) 21 having a waveform can be easily formed, and the microphone 20 having high sensitivity can be constructed. In particular, since a plurality of microphones 20 can be manufactured collectively and with uniform quality on a Si wafer, and the microphones 20 can be formed integrally with the acoustic cell, the industrial advantage is enormous. is there.

The present invention is not limited to the above embodiment. For example, as shown in FIG. 4A, a plurality of arc-shaped concave portions 14a are formed on the Si substrate 11 by isotropic etching.
And a microphone 20 having a structure as shown in FIG. 4B may be realized based on the waveform formed by the arc-shaped concave portions 14a. Further, as shown in FIG. 5A, a second electrode 22 and a back plate 23 supporting the second electrode 22 are formed in a waveform shape following the first electrode 21, and Au is formed on the back plate 23. Alternatively, the infrared light reflection film 18 made of, for example, may be formed. However, in this case, as shown in FIG. 5B, it is preferable to provide the cavity 10 on the back plate 23 side and to irradiate the infrared light into the cavity 10 from above in the figure. . At this time, the S electrode used for forming the first electrode (diaphragm) 21 was used.
It goes without saying that the i-substrate 11 is etched from the back side to expose the first electrode (diaphragm) 21. The material for forming the cavity 10 is Si-
Not only Si but also Si and glass can be used.

The infrared light reflecting film 18 may be made of Al or the like, and the material for forming the first electrode 21 may be made of a noble metal material having an infrared light reflecting function. . Furthermore, the size of the first electrode 21, the depth of the surface forming the waveform, the pitch of the waves, and the like may be determined according to the detection sensitivity specifications required for the microphone 20 and the acoustic cell. Further, instead of the Si substrate 11, a cavity can be formed using another semiconductor substrate. In addition, the present invention can be variously modified and implemented without departing from the gist thereof.

[0027]

As described above, according to the present invention, it is possible to realize a microphone for a photoacoustic gas sensor whose detection sensitivity is enhanced because the first electrode forming a part of the cavity has a waveform. In addition, since the first electrode itself can have a function of reflecting infrared light, the gas detection sensitivity of the acoustic cell can be easily increased.
Moreover, since the microphone is manufactured by etching the semiconductor substrate forming the cavity and defining the waveform shape of the first electrode, the manufacture is extremely simple and has a great effect in practical use such as excellent mass productivity. Can be

[Brief description of the drawings]

FIG. 1 is a diagram showing a step-by-step disassembly of a manufacturing process of a microphone for a photoacoustic gas sensor according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a manufacturing process of the microphone for the photoacoustic gas sensor following FIG. 1 and a schematic structure of the microphone manufactured by the process.

FIG. 3 is a view showing an example of the shape of a hole formed in a thermal oxide film used as a mask for anisotropic etching of a Si substrate.

FIG. 4 is a diagram showing a main part of a manufacturing process of a microphone for a photoacoustic gas sensor according to another embodiment of the present invention, and a structure of the microphone.

FIG. 5 is a diagram showing a main part of a manufacturing process of a microphone for a photoacoustic gas sensor according to still another embodiment of the present invention, and a structure of the microphone.

FIG. 6 is a diagram showing a schematic structure of a photoacoustic gas sensor.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Cavity 11 Si substrate 12 Thermal oxide film (mask) 14 Concave portion (wave shape) 16 Sacrificial layer 17 Thermal oxide film 18 Infrared light reflection film 20 Microphone 21 First electrode (wave shape) 22 Second electrode 23 Back plate 24 sound holes

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Nobuaki Honda 2-12-19 Shibuya, Shibuya-ku, Tokyo F-term (reference) 2F055 AA39 BB20 CC02 DD05 EE25 FF11 GG01 GG15 HH05 HH19 2G047 AA01 CA04 GB11 GB32 GD02 4M112 AA01 AA03 BA07 CA03 CA04 CA11 CA13 DA04 EA03 EA14 FA01

Claims (6)

[Claims] 1. A microphone provided as a part of a cavity into which a gas is introduced and responsive to a pressure in the cavity, comprising: a first electrode having a corrugated shape forming a part of a wall surface of the cavity; A microphone for a photoacoustic gas sensor, comprising: a second electrode supported by a back plate connected to the cavity and opposed to the first electrode at a predetermined distance. 2. The semiconductor device according to claim 1, wherein the cavity is formed by forming a through hole having a predetermined opening area in the semiconductor substrate, and wherein the first electrode closes one opening of the through hole. The microphone for a photoacoustic gas sensor according to claim 1, which is provided. 3. The microphone for a photoacoustic gas sensor according to claim 1, wherein the first electrode or the back plate includes an infrared light reflecting film coated on a surface exposed to the cavity. 4. A method of manufacturing a microphone provided as a part of a cavity into which a gas is introduced and responding to pressure in the cavity, wherein one surface of the semiconductor substrate is formed using a mask having a predetermined opening pitch. A first step of etching and processing one surface of the semiconductor substrate into a corrugated shape; a second step of forming a first electrode along the one surface of the semiconductor substrate processed into the corrugated shape; A third step of forming a sacrifice layer having a predetermined thickness on the upper surface of the first electrode, a fourth step of forming a second electrode on the sacrifice layer, and a back plate covering the second electrode Forming a predetermined opening by patterning the back plate to form an acoustic hole; and selectively etching the semiconductor substrate from the back side thereof to form the first electrode. Exposed 6. A method for manufacturing a microphone for a photoacoustic gas sensor, comprising: a sixth step; and a seventh step of etching and removing the sacrificial layer through the acoustic hole. 5. The method for manufacturing a microphone for a photoacoustic gas sensor according to claim 4, further comprising coating an infrared light reflecting film on a surface of the first electrode or the back plate exposed to the cavity. 8. A method for manufacturing a microphone for a photoacoustic gas sensor, comprising: 6. The method for manufacturing a microphone for a photoacoustic gas sensor according to claim 4, wherein the first step includes anisotropically etching one surface of the semiconductor substrate.